July i, 1921 
Pathogenicity of Corticium vagum on the Potato 
479 
of the young shoots with concomitant increased rate of cellular differen¬ 
tiation enters in as an important factor in lessening the degree of injury 
at the higher temperatures. Again, the depth and the spread of lesions 
may be closely associated with the growing-point injury which is most 
severe at the lower temperatures. On the other hand, it is very probable 
that decreased tissue destruction is in part at least a result of a direct 
inhibitive action of the higher temperature upon the physiological activi¬ 
ties of the fungus. Balls ( 2 ) noted a similar decreased parasitic activity 
of “sore-shin fungus" (Rhizoctonia solani) on the cotton plant and 
postulated an inhibiting factor “ X " in the form of a by-product of fungous 
metabolism. He assumes that this accumulates at the higher tempera¬ 
tures and limits both infection and subsequent action of the fungus on 
the cotton tissues. Whether or not such explanation is justified, it is 
evident that it does not apply to the inhibited parasitic activity which 
resulted in a lesser degree at temperatures below 18 0 . A determination 
of the type and the rate of enzym secretion by the fungus at the various 
temperatures would undoubtedly aid greatly in understanding the 
problem. 
The relation of the temperature of the soil to the destruction of the 
primordia of the young shoots appears more clear. Evidence has accumu¬ 
lated which indicates that this type of injury depends largely upon an 
opportunity for the mycelium to accumulate in the sinus of the bud of 
the young stem in contact with the delicate primordium (Pi. 89, A). At 
the lower temperatures the rate of growth of the young shoots is so 
retarded as to increase definitely the time of exposure of the young buds 
to the action of the fungus in the soil. The opportunity for the accumu¬ 
lation of the mycelium in the growing points and for subsequent tissue 
destruction is thereby greatly increased. 
The increase in the number of stems noted at the lower temperatures 
is doubtless closely associated with the growing-point destruction and 
must be considered as the result of a peculiar balance between the host 
and the parasite which is ultimately conditioned by the temperature of 
the soil. Granted the presence of a virulent “strain" of Corticium 
vagum , it appears evident that the number of stems which survive and 
finally appear through the soil in an infested area will depend to a large 
extent upon the duration of temperatures favorable or unfavorable for 
the pathogenic operation of the fungus. When a favorable temperature 
is maintained for a considerable period not only will large numbers of 
primary growing points be destroyed but secondary and even tertiary 
buds may succumb to the attack of the fungus, thus decreasing the 
number of successful stems per hill. When, however, the period of soil 
temperature favorable for pathogenicity of the fungus is short, resulting 
secondary stems may escape injury; and as two or even more may start 
from a single injured sprout, a decided increase in the number of stems is 
